Monolayer graphene exhibits impressive in-plane thermal conductivity (> 1000Wm–1K–1). However, the out-of-plane thermal transport is limited due to the weak van der Waals interaction, indicating the possibility of constructing a vertical thermoelectric (TE) device. Here, we propose a cross-plane TE device based on the vertical heterostructures of few-layer graphene and gold nanoparticles (AuNPs) on Si substrates, where the incorporation of AuNPs further inhibits the phonon transport and enhances the electrical conductivity along vertical direction. A measurable Seebeck voltage is produced vertically between top graphene and bottom Si when the device is put on a hot surface and the figure of merit ZT is estimated as 1 at room temperature from the transient Harman method. The polarity of the output voltage is determined by the carrier polarity of the substrate. The device concept is also applicable to a flexible and transparent substrate as demonstrated. Display omitted •A new TE device of few-layer graphene with intercalated Au nanoparticles.•The ueage of Au nanoparticles enhances the cross-plane electrical conductivity.•The figure of merit ZT is estimated as 1 at room temperature.•The polarity of output voltage is determined by the carrier polarity of the substrate.•The device concept is applicable to a flexible and transparent substrate.